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Merge pull request #478 from iluwatar/DocumentDoubleCheckIdiom

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Documented singleton double check idiom
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Ilkka Seppälä 2016-08-05 18:45:00 +03:00 committed by GitHub
commit d50139e13e
1 changed files with 9 additions and 0 deletions
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singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java
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@ -53,11 +53,20 @@ public final class ThreadSafeDoubleCheckLocking {
public static ThreadSafeDoubleCheckLocking getInstance() { public static ThreadSafeDoubleCheckLocking getInstance() {
// local variable increases performance by 25 percent // local variable increases performance by 25 percent
// Joshua Bloch "Effective Java, Second Edition", p. 283-284 // Joshua Bloch "Effective Java, Second Edition", p. 283-284
ThreadSafeDoubleCheckLocking result = instance; ThreadSafeDoubleCheckLocking result = instance;
// Check if singleton instance is initialized. If it is initialized then we can return the instance.
if (result == null) { if (result == null) {
// It is not initialized but we cannot be sure because some other thread might have initialized it
// in the meanwhile. So to make sure we need to lock on an object to get mutual exclusion.
synchronized (ThreadSafeDoubleCheckLocking.class) { synchronized (ThreadSafeDoubleCheckLocking.class) {
// Again assign the instance to local variable to check if it was initialized by some other thread
// while current thread was blocked to enter the locked zone. If it was initialized then we can
// return the previously created instance just like the previous null check.
result = instance; result = instance;
if (result == null) { if (result == null) {
// The instance is still not initialized so we can safely (no other thread can enter this zone)
// create an instance and make it our singleton instance.
instance = result = new ThreadSafeDoubleCheckLocking(); instance = result = new ThreadSafeDoubleCheckLocking();
} }
} }